Numbness after surgery

I got home from hospital yesterday after partial mastectomy and sentinel lymph node removal.  To be honest, I felt so knocked around that I didn't know which way was up.  While washing in a pathetically low bath this morning, I realised that the whole area under my arm, halfway to the elbow, is completely numb to the touch.  Not what I was expecting.  I'm assuming it's some sort of nerve freakout.  Is this normal??? and is it likely to get better?

By the way, I took someone's advice ( @primek ?) and bought a couple of the Kmart pull-on bras while I was out rushing around doing the Chrissie shopping before I went in for surgery.  Tried one this morning, and much as I appreciate Berlei and their free body armour (and I will use it when I need it), it's so much better to be in something less constricting.